System and method for integrating energy storage into modular power converter

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

0001This invention was made with Government support under contract number N00014-14-C-0103 awarded by the Office of Naval Research. The Government has certain rights in the invention.

BACKGROUND

0002Embodiments of the invention relate power converters and more specifically to a system and a method for integrating energy storage into modular multi-level power converter.

0003Energy storage has becoming an increasingly important key element for a variety of power system applications, including motive/transportation and stationary/power grid applications. Traditionally, electrical energy storage such as batteries and ultracapacitors are designed and well suited for direct current (DC) applications that provide active power and load support. Often times, DC to DC power converter will be needed in coupling the energy storage module(s)/element(s) to a DC bus in the power system. For an alternating current (AC) application (e.g., supporting AC loads), the DC bus will then be tied to an AC power system/grid via a DC/AC inverter. This requires multiple stages of power conversion, associated with power conversion efficiency/loss at each stage.

0004For the DC/AC power conversion, modular multi-level converters (MMCs) are becoming more and more utilized in particular for high and medium voltage applications due to its modular and scalable characteristics. MMCs also have high reliability through the use of redundant modules and high output power quality (e.g., less total harmonic distortion/THD) with little or no filters, thereby reducing system weight and volume or increasing power density.

0005When MMC is integrated with additional energy storage units, such as batteries and/or super-capacitors/ultracapacitors, it reduces the number of power conversion stages and thus increases overall system efficiency. In addition, when MMC is combined with multiple (or hybrid) energy storage modules in one system, it provides multiple system functions such as uninterrupted power supply (UPS) and transient assist/smoothing functions.

0006The challenge of using energy storage units (e.g., super-capacitors) within the MMC is the limitation in the amount of current ripple that the energy storage units (e.g., ultracapacitors) can accept without significantly overheating; reducing their useful life; and/or significantly oversizing the energy storage elements. For instance, when the module ultracapacitors are used for a 60 Hz application, they have to conduct significant ripple currents with 60 Hz, 120 Hz, and other frequency content. This oversized energy storage capacity to handle the ripple currents do not support active power/load, and significantly increase/penalize system volume and weight.

0007For these and other reasons, there is a need for improved system for integrating energy storages with modular multilevel converters.

BRIEF DESCRIPTION

0008In accordance with an embodiment of the present technique, a system for integrating energy storage into a modular power converter is presented. The modular converter includes at least one energy storage unit coupled to a first converter for converting a first direct current (DC) voltage of the at least one energy storage unit into a first high frequency alternating current (AC) voltage. The system also includes at least three phase legs of the modular power converter for generating three phase AC voltages, wherein each phase leg includes a plurality of switching modules connected in series. Each switching module comprises a plurality of fully controllable semiconductor switches, an energy storage device and a second converter coupled to the respective energy storage devices for converting a second DC voltage of the energy storage device into a second high frequency AC voltage. One power unit is formed by three similarly positioned switching modules of the three phase legs. The system further includes a high frequency transformer having at least one primary winding connected to the first converter and at least three secondary windings, each connected to the second converter of each of the three similarly positioned switching modules. The system also includes a controller configured to regulate at least one electrical parameter of the modular power converter.

0009In accordance with another embodiment of the present technique, a method for integrating energy storage into a modular power converter is provided. The method includes coupling at least one energy storage unit to a first converter, wherein the first converter converts a first direct current (DC) voltage of the energy storage unit into a first high frequency alternating current (AC) voltage. The method also includes generating three phase AC voltages from at least three phase legs of the modular power converter, each phase leg including a plurality of switching modules connected in series, wherein each of the switching module includes a plurality of fully controllable semiconductor switches, an energy storage device and a second converter coupled to the respective energy storage device for converting a second DC voltage of the energy storage device into a second high frequency AC voltage. The method further includes forming one power unit from the three similarly positioned switching modules of the three phase legs. Furthermore, the method includes providing a high frequency transformer having at least one primary winding and at least three secondary windings between the power unit and the first power converter, wherein the primary winding is connected to the first power converter and each of the three secondary windings is connected to the second converter of each of the three similarly positioned switching modules. The method also includes regulating at least one electrical parameter of the modular power converter

DRAWINGS

0010<figref idref="DRAWINGS">FIG. 1</figref> is a diagrammatical representation of a system for power conversion;

0011<figref idref="DRAWINGS">FIG. 2</figref> is a diagrammatical representation of a system for integrating an energy storage unit into a modular power converter, in accordance with an embodiment of the present technique;

0012<figref idref="DRAWINGS">FIG. 3</figref> is a diagrammatical representation of a system for integrating an energy storage unit into a modular power converter, in accordance with another embodiment of the present technique;

0013<figref idref="DRAWINGS">FIG. 4</figref> is a diagrammatical representation of a system for integrating energy storage units into a modular multilevel converter, in accordance with an embodiment of the present technique; and

0014<figref idref="DRAWINGS">FIG. 5</figref> s a diagrammatical representation of a system for integrating energy storage units into a flexible AC transmission system (FACTs) device, in accordance with an embodiment of the present technique.

DETAILED DESCRIPTION

0015Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this disclosure belongs. The terms “first”, “second”, and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The term “or” is meant to be inclusive and mean one, some, or all of the listed items. The use of “including,” “comprising” or “having” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “connected” and “coupled” are not restricted to physical or mechanical connections or couplings, and can include electrical connections or couplings. The connections may include direct connections. Furthermore, the terms “circuit” and “circuitry” and “controller” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together to provide the described function.

0016The embodiments of the present technique propose a new power conversion topology that cancels or eliminates the current ripples going through the energy storage units used in conjunction with modular power converters and also provide safety and protection via galvanic isolation.

0017The technique is to combine the energy storage units from the three legs that are at the same relative position (in other words, similarly positioned) within phase arms into one by integrating a direct current (DC) transformer (DC to DC converter a high frequency transformer) with 4 sets of windings. Three of the transformer windings will be coupled to three switching modules from three phase legs and the fourth transformer winding will couple to the energy storage unit. Thus, the energy storage units will only carry the sum of the balanced three 120 degrees phase shifted modules currents, which when added will cancel the alternating current (AC) ripple currents and result in a DC current going through the energy storage element.

0018Turning now to the drawings, by way of example in <figref idref="DRAWINGS">FIG. 1</figref>, a system <b>100</b> for converting power is depicted. In one embodiment, the system <b>10</b> for converting power may include a source <b>12</b>, a power converter <b>14</b>, and a grid/utility/load <b>16</b>. The term source, as used herein, is used to refer to a renewable power source, a non-renewable power source, a generator, a grid, and the like. Also, the term load, as used herein, may be used to refer to a grid, an electrical appliance, and the like. In addition, the power converter <b>14</b> may be a modular power converter. In one embodiment, the source <b>12</b> may be operatively coupled to a first terminal (not shown) of the power converter <b>14</b>. A second terminal (not shown) of the power converter <b>14</b> may be operatively coupled to the load <b>16</b>. The first terminal and the second terminal may be alternatively employed as an input terminal or an output terminal of the power converter <b>14</b>. The term operatively coupled, as used herein, may include wired coupling, electrical coupling, magnetic coupling, and the like.

0019Also, the system <b>10</b> may include a controller <b>18</b>. The controller <b>18</b> may be configured to control the operation of the power converter <b>14</b>, in one embodiment. By way of example, the controller <b>18</b> may be configured to control the operation of the power converter <b>14</b> by controlling switching of a plurality of semiconductor switches of the power converter <b>14</b>. Furthermore, in one embodiment, the system <b>10</b> may also include other circuit components (not shown) such as, but not limited to, a transformer, a circuit breaker, an inductor, a compensator, a capacitor, a rectifier, a reactor, a filter, and the like. In one embodiment, of the present technique, an energy storage unit may be coupled to the power converter <b>14</b> or may be a part of it as will be explained in greater detail with respect to <figref idref="DRAWINGS">FIGS. 2 and 4</figref>.

0020<figref idref="DRAWINGS">FIG. 2</figref> shows a system <b>100</b> for integrating an energy storage unit into a modular power converter, in accordance with an embodiment of the present technique. The system <b>100</b> includes an energy storage unit <b>102</b> coupled to a first converter <b>104</b>. The energy storage unit <b>102</b> may include batteries, flywheels, ultracapacitors, or supercapacitors. The batteries may be of type such as Li-ion, sodium-chloride, Lithium Titanium (LTO), Lithium Nickel Manganese Cobalt Oxide (NMC), or metal-air to name a few. The energy storage unit may further include fuel cells or other capacitors such as film capacitors or electrolytic capacitors. Further, the first converter <b>104</b> includes a DC to high frequency AC converter which converters a first DC voltage V<b>1</b> of the energy storage unit <b>102</b> into a first high frequency AC voltage Vp. The first converter <b>104</b> includes a full bridge converter having two legs, each having two fully controllable semiconductor switches whose terminals are connected in series. The fully controllable semiconductor switches may include an insulated gate bipolar transistor (IGBT), a metal oxide semiconductor field effect transistor (MOSFET), a field effect transistor (FET), a gate turn-off thyristor, an insulated gate commutated thyristor (IGCT), an injection enhanced gate transistor (IEGT), a silicon carbide based switch, a gallium nitride based switch, a gallium arsenide based switch, or equivalents thereof. The full bridge converter can reverse the polarity of output voltage (i.e., Vp) and output current. The high frequency voltage Vp is applied across a primary winding <b>106</b> of a high frequency transformer <b>108</b>. The term ‘high frequency’ refers to any frequency greater than 5 kHz.

0021Furthermore, the system <b>100</b> includes a modular power converter <b>110</b> having three phase legs <b>112</b>, <b>114</b> and <b>116</b> for phases a, b and c respectively. Phase leg <b>112</b> includes a first switching module <b>118</b> and switching modules (2 to n) <b>120</b>, all of which are connected in series. Similarly, phase leg <b>114</b> includes a first switching modules <b>122</b> and switching modules (2 to n) <b>124</b> connected in series. Further, phase leg <b>116</b> includes a first switching module <b>126</b> and switching modules (2 to n) <b>128</b> connected in series.

0022Switching module <b>118</b> includes an energy storage device <b>132</b> connected in parallel with a set <b>134</b> of two controllable semiconductor switches. The terminals of the two controllable semiconductor switches are connected in series. The terminals of the energy storage device <b>132</b> are further connected in parallel with a second converter <b>136</b>. The second converter <b>136</b> includes a DC to high frequency AC converter which converters a second DC voltage V<b>21</b> of the energy storage device <b>132</b> into a second high frequency AC voltage Vs<b>1</b> applied across a first secondary winding <b>138</b> of the high frequency transformer <b>108</b>. It should be noted that other series connected switching modules (2 to n) <b>120</b> of phase leg <b>112</b> are similar in structure to switching module <b>118</b>. Also it should be noted that the series connection of switching modules only refers to terminals of set <b>134</b> of two controllable semiconductor switches of switching modules <b>118</b>, <b>120</b> being connected in series. The second voltages Vs<b>1</b> of switching modules of one phase leg are neither connected in series nor are they connected to the same transformer.

0023Similar to switching module <b>118</b>, switching module <b>122</b> includes an energy storage device <b>140</b> connected in parallel with a set <b>142</b> of two controllable semiconductor switches. The terminals of the energy storage device <b>140</b> are further connected in parallel with a second converter <b>144</b>. The second converter <b>144</b> includes a DC to high frequency AC converter which converters a second DC voltage V<b>22</b> of the energy storage device <b>140</b> into a second high frequency AC voltage Vs<b>2</b> applied across a second secondary winding <b>146</b> of the high frequency transformer <b>108</b>.

0024Further, switching module <b>126</b> includes an energy storage device <b>148</b> connected in parallel with a set <b>150</b> of two controllable semiconductor switches. The terminals of the energy storage device <b>148</b> are further connected in parallel with a second converter <b>152</b>. The second converter <b>152</b> includes a DC to high frequency AC converter which converters a second DC voltage V<b>32</b> of the energy storage device <b>148</b> into a second high frequency AC voltage Vs<b>3</b> applied across a third secondary winding <b>154</b> of the high frequency transformer <b>108</b>.

0025As with first converter <b>104</b>, second converters <b>136</b>, <b>144</b> and <b>152</b> may include full bridge converters having two legs connected in parallel to energy storage unit <b>102</b>. Each leg of full bridge converter <b>104</b> includes two fully controllable semiconductor switches whose terminals are connected in series. The interconnections of two fully controllable semiconductor switches form an output connection. Further, one side end terminals of phase legs <b>112</b>, <b>114</b> and <b>116</b> may be connected to each other and/or another side end terminals may provide three phase AC output voltages. The similarly positioned switching modules of all the phase legs such as first switching modules <b>118</b>, <b>122</b> and <b>126</b> of three phase legs <b>112</b>, <b>114</b> and <b>116</b> respectively together form one power unit <b>156</b> of modular power converter <b>110</b>. Every energy storage unit integrated with the modular power converter is coupled with one such one power unit. For example, energy storage unit <b>102</b> and related first power converter <b>104</b> is coupled to power unit <b>156</b> via high frequency transformer <b>108</b>.

0026The system <b>100</b> also includes a controller <b>158</b> which controls the operation of first converter <b>104</b>, second converters <b>136</b>, <b>144</b> and <b>152</b> such that voltages V<b>21</b>, V<b>22</b> and V<b>23</b> of energy storages devices <b>132</b>, <b>140</b> and <b>148</b> may be maintained. As discussed earlier, in one embodiment, first and second converters <b>104</b>, <b>136</b>, <b>144</b> and <b>152</b> may be full bridge converters and therefore, energy may be transferred from energy storage devices <b>132</b>, <b>140</b> and <b>148</b> to energy storage unit <b>102</b> or from energy storage unit <b>102</b> to energy storage devices <b>132</b>, <b>140</b> and <b>148</b>.

0027Further, in case of a fault on either energy storage unit <b>102</b> or modular power converter <b>110</b>, the controller may send controls signals to first and second power converters <b>104</b>, <b>136</b>, <b>144</b> and <b>152</b> such that the two converters on either side of the transformer <b>108</b> may be isolated. The winding turns ratio of primary winding <b>106</b> and secondary windings <b>138</b>, <b>146</b> and <b>154</b> may be 1 to 1 or it could be any other turns ratio based on system operation requirement. This flexibility of winding turns ratio helps to select voltages at energy storage devices relatively independent of voltages at energy storage units, while preserving the overall system efficiency. The frequency of voltages Vs<b>1</b>, Vs<b>2</b>, Vs<b>3</b> and Vp may be higher than 5 kHz.

0028It should be noted that in another embodiment, transformer <b>108</b> may include more than three secondary windings. For example, transformer <b>108</b> along with secondary windings <b>138</b>, <b>146</b> and <b>154</b> may include at least three more secondary windings for the subsequent similarly positioned switching modules.

0029The controller <b>158</b> further controls at least one electrical parameter of modular power converter <b>110</b>. The electrical parameter may include output current or output voltage or output power of the modular power converter <b>110</b>. It should be noted that since the currents in legs <b>112</b>, <b>114</b> and <b>116</b> are from three different phases (i.e., phases a, b and c) of the modular converter <b>110</b>, the currents are 120 degrees phase shifted. These 120 degrees phase shifted currents then flow through the three secondary windings <b>138</b>, <b>146</b> and <b>154</b>. However, on the primary side there is only one winding which carries sum of these balanced three 120 degrees phase shifted currents. As the sum of 120 degree phase shifted currents is zero, the AC ripple currents will cancel out and only DC current will flow through the energy storage unit <b>102</b>. It should also be noted that due to the transformer isolation between energy storage unit and the energy storage devices, in one embodiment, the energy storage unit <b>102</b> could also be electrically grounded, which could be advantageous if energy storage unit is used in parallel with batteries. As shown in <figref idref="DRAWINGS">FIG. 2</figref>, in one embodiment, the energy storages devices may also be floated.

0030<figref idref="DRAWINGS">FIG. 3</figref> shows a system <b>200</b> for integrating an energy storage unit into a modular power converter, in accordance with another embodiment of the present technique. As compared with system <b>100</b>, system <b>200</b> includes half bridge converters for first and second converters instead of full bridge converters. The system <b>200</b> includes an energy storage unit <b>202</b> coupled to a first converter <b>204</b>. The first converter <b>204</b> includes a half bridge converter having two legs connected in parallel across the energy storage unit <b>202</b>. One converter leg of converter <b>204</b> includes two fully controllable semiconductor switches whose terminals are connected in series and another leg includes two capacitors connected in series. The interconnections of two fully controllable semiconductor switches and two capacitors form two output terminals of converter <b>204</b>. The half bridge converter <b>204</b> can reverse the polarity of output voltage (i.e., Vp) but not of output current. The output high frequency AC voltage of converter <b>204</b> is applied across a primary winding <b>206</b> of a high frequency transformer. It should be noted that although the primary winding <b>206</b> and secondary windings <b>208</b>, <b>210</b> and <b>212</b> are shown to be located apart from each other, they are part of a single transformer.

0031Furthermore, the system <b>200</b> three phase legs <b>214</b>, <b>216</b> and <b>218</b> of a modular converter. Phase leg <b>214</b> includes a plurality of switching modules connected in series but only first switching module <b>220</b> is shown herewith for ease of explanation. Similarly, phase leg <b>216</b> includes a first switching module <b>222</b> and phase leg <b>218</b> includes a first switching module <b>224</b>.

0032Switching module <b>220</b> includes a second converter <b>226</b>, output terminals of which are connected across secondary winding <b>208</b>. Similarly, switching module <b>222</b> includes a second converter <b>228</b>, output terminals of which are connected across secondary winding <b>210</b> and switching module <b>224</b> includes a second converter <b>230</b>, output terminals of which are connected across secondary winding <b>212</b>. The second converter <b>226</b>, <b>228</b> and <b>230</b> include half bridge converters similar to first converter <b>204</b>. As with embodiment of <figref idref="DRAWINGS">FIG. 2</figref>, switching modules <b>220</b>, <b>222</b>, and <b>224</b> also include controllable semiconductor switches connected in series. A controller then controls these first and second converters to maintain voltage across energy storage devices of switching modules <b>220</b>, <b>222</b>, and <b>224</b>.

0033<figref idref="DRAWINGS">FIG. 4</figref> shows a system <b>300</b> for integrating energy storage units into a modular multilevel converter, in accordance with an embodiment of the present technique. System <b>300</b> shows three legs <b>302</b>, <b>304</b> and <b>306</b> of a modular multilevel converter (MMC) <b>301</b>, wherein each leg has a plurality of switching modules connected in series. As discussed earlier, the series connection of switching modules here refers to terminals of set of controllable semiconductor switches of switching modules being connected in series. Further system <b>300</b> includes energy storage units <b>308</b>, <b>310</b>, <b>312</b>, <b>314</b>, <b>316</b> and <b>318</b> which are coupled respectively to power units <b>320</b>, <b>322</b>, <b>324</b>, <b>326</b>, <b>328</b> and <b>330</b> of modular multilevel converter via high frequency transformers. As explained earlier, power units <b>320</b>, <b>322</b>, <b>324</b>, <b>326</b>, <b>328</b> and <b>330</b> of modular multilevel converter are formed by similarly positioned switching modules of phase legs <b>302</b>, <b>304</b> and <b>306</b>. For example, in the embodiment shown, every phase leg <b>302</b>, <b>304</b> or <b>306</b> has six switching modules connected in series. If the switching modules are numbered 1 to 6 respectively then, number <b>1</b> switching modules of all three legs form one power module. Similarly, number <b>2</b> switching modules of all three legs form another power module and so on.

0034The end terminals (i.e., starting terminals <b>332</b>, <b>334</b>, <b>336</b> and ending terminals <b>338</b>, <b>340</b>, <b>342</b>) of converter phase legs <b>302</b>, <b>304</b> and <b>306</b> are connected together. The starting terminals <b>332</b>, <b>334</b>, <b>336</b> when connected together form a positive DC bus and ending terminals <b>338</b>, <b>340</b>, <b>342</b> when connected together form a negative DC bus. A DC voltage source <b>344</b> is then connected across the positive DC bus and the negative DC bus. The DC voltage source <b>344</b> may be a HVDC bipolar line coming from another substation or it may be a DC link capacitor.

0035The switching modules of every leg <b>302</b>, <b>304</b> or <b>306</b> of modular multilevel converter are further separated into a first arm and a second arm. For example, switching modules of phase leg <b>302</b> are separated into a first arm <b>346</b> and a second arm <b>348</b>. The first arm <b>346</b> and the second arm <b>348</b> are connected to each other by inductors <b>350</b> and <b>352</b> as shown. Similarly, switching modules of phase leg <b>304</b> are separated into a first arm <b>354</b> and a second arm <b>356</b>. The first arm <b>354</b> and the second arm <b>356</b> are connected to each other by inductors <b>358</b> and <b>360</b>. Further, switching modules of phase leg <b>306</b> are separated into a first arm <b>362</b> and a second arm <b>364</b>. The first arm <b>362</b> and the second arm <b>364</b> are connected to each other by inductors <b>366</b> and <b>368</b>. The interconnection points <b>370</b>, <b>372</b> and <b>374</b> of first arms <b>346</b>, <b>354</b>, <b>362</b> and second arms <b>348</b>, <b>356</b>, <b>364</b> via the respective inductors provide three phase output terminals <b>370</b>, <b>372</b>, <b>374</b> of modular multilevel converter <b>301</b>. The terminals <b>370</b>, <b>372</b>, <b>374</b> may further provide three phase AC voltages to a power grid or a load.

0036A controller (e.g. controller <b>158</b> of <figref idref="DRAWINGS">FIG. 2</figref>) controls the system <b>300</b> in such a way that the energy storage devices of switching modules are charged by either DC voltage source or the power grid. As discussed earlier, the controller also controls first converters and second converters such that the voltage of energy storage devices or energy storage units may be controlled. Further, the controller may control the modular multilevel converter in such a way that the three phase output terminals electrical parameters and/or the positive bus and negative bus electrical parameters are regulated.

0037<figref idref="DRAWINGS">FIG. 5</figref> shows a system <b>400</b> for integrating energy storage units into a flexible AC transmission system (FACTs) device, in accordance with an embodiment of the present technique. System <b>400</b> shows three legs <b>404</b>, <b>406</b> and <b>408</b> of a modular converter <b>402</b> which may be employed in a FACTs device such as a static compensator (STATCOM). Each leg has a plurality of switching modules connected in series. Further, system <b>400</b> includes energy storage units <b>410</b>, <b>412</b>, <b>414</b> and <b>416</b> which are coupled respectively to power units <b>418</b>, <b>420</b>, <b>422</b> and <b>424</b> of modular converter <b>402</b> via high frequency transformers. Power units <b>418</b>, <b>420</b>, <b>422</b> and <b>424</b> of modular converter are formed by similarly positioned switching modules of phase legs <b>404</b>, <b>406</b> and <b>408</b> respectively.

0038In the embodiment shown, one side end terminals <b>426</b>, <b>428</b>, <b>430</b> of converter phase legs <b>404</b>, <b>406</b> and <b>408</b> are connected together and form a floating bus. The other side terminals <b>432</b>, <b>434</b> and <b>436</b> of converter phase legs <b>404</b>, <b>406</b> and <b>408</b> provide three phase AC output terminals of modular multilevel converter <b>402</b>. The terminals <b>432</b>, <b>434</b> and <b>436</b> are further connected to a power grid and the converter <b>402</b> can be used to compensate for reactive power or for increasing stability of power grid.

0039The controller controls the system <b>400</b> in such a way that the energy storage devices of switching modules are charged by the power grid. The controller also controls first converters and second converters such that the voltage of energy storage devices or energy storage units may be controlled. Further, the controller may control the modular multilevel converter in such a way that the three phase output terminals electrical parameters and/or the positive bus and negative bus electrical parameters are regulated.

0040It should be noted that embodiments shown in <figref idref="DRAWINGS">FIGS. 4 and 5</figref> are only two example applications of the present technique. However, the present technique of integrating energy storage units may also be utilized in any other type of modular converters such as modular embedded multilevel converters and the like.

0041While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.